Along with paella, the ubiquitous Spanish omelette - tortilla de patatas - is perhaps one of the best-known Spanish dishes. It is impossible to find a self-respecting tapas bar that does not feature tortilla in its repertoire.
As delicious as it is versatile, this Spanish staple lends itself to countless variations according to personal taste. Some cooks mix in mushrooms, beans, spinach, and tomatoes, while others choose to omit the onion and instead cover the tortilla in tomato sauce. Others still would never dream of serving the tortilla without heaping mounds of mayonnaise. Each region, and each tapas bar, will have its own variation of the traditional tortilla. This delicious tapa can be served warm or cold.
Heat the olive oil in a 9-inch skillet and add the potato slices carefully, because the salt will make the oil splatter. Try to keep the potato slices separated so they will not stick together. Cook, turning occasionally, over medium heat for 5 minutes. Add the onions and garlic and cook until the potatoes are tender. Drain into a colander, leaving about 3 tablespoons of oil in the skillet.
Meanwhile, in a large bowl, whisk the eggs with a pinch of salt. Add the potatoes, and stir to coat with the egg. Add the egg-coated potatoes to the very hot oil in the skillet, spreading them evenly to completely cover the base of the skillet. Lower the heat to medium and continue to cook, shaking the pan frequently, until mixture is half set.
Use a plate to cover the skillet and invert the omelette away from the hand holding the plate (so as not to burn your hand with any escaping oil). Add 1 tablespoon oil to the pan and slide the omelette back into the skillet on its uncooked side. Cook until completely set. Allow the omelette to cool, and then cut it into wedges. Season it with salt and sprinkle with lemon juice to taste (optional).
Serve warm or at room temperature.